JP6579812B2 - Fixing device - Google Patents

Description

  Embodiments described herein relate generally to a fixing device.

  Conventionally, there are image forming apparatuses such as a Multi Function Peripheral (hereinafter referred to as “MFP”) and a printer. The image forming apparatus includes a fixing device. The fixing device heats the conductive layer of the fixing belt by an electromagnetic induction heating method (hereinafter referred to as “IH method”). The fixing device fixes the toner image on the recording medium by the heat of the fixing belt. For example, the fixing device includes a heat generation auxiliary unit that faces the induced current generation unit with the fixing belt interposed therebetween. The induction current generating unit generates a magnetic flux by applying a high frequency current from the inverter drive circuit. The inverter drive circuit includes an IGBT (Insulated Gate Bipolar Transistor) element. The heat generation assisting unit concentrates the magnetic flux during electromagnetic induction heating and increases the heat generation amount of the fixing belt. Here, the heat generation of the fixing belt is adjusted by power control. In order to keep the temperature of the fixing belt (hereinafter referred to as “belt temperature”) constant, the induction current generator is controlled to a constant output. When the heat generating auxiliary part is formed of a magnetic shunt alloy, the heat generating auxiliary part loses magnetism when the Curie point is exceeded and does not form a magnetic path. If the magnetic path is not formed by the heat generation assisting unit, the load (electric resistance) of the induced current generating unit is reduced. The fixing device increases the current flowing through the inverter drive circuit by the amount of decrease in the load of the induced current generator, and tries to keep the output of the induced current generator constant. When the current flowing through the inverter drive circuit increases, the current flowing through the IGBT element also increases, which may excessively increase the temperature of the IGBT element and damage the IGBT element.

JP 2006-47768 A

  The problem to be solved by the present invention is to provide a fixing device capable of preventing the IGBT element from being damaged.

  The fixing device according to the embodiment includes a fixing belt, an induction current generation unit, a heat generation auxiliary unit, a measurement unit, and a control unit. The fixing belt includes a conductive layer. The induced current generator is opposed to the fixing belt. The induction current generator includes a first coil that electromagnetically heats the conductive layer. The heat generation assisting portion faces the induced current generating portion with the fixing belt interposed therebetween. The heat generating auxiliary part has a magnetic material having a Curie point lower than that of the conductive layer. The measurement unit measures the state of the heat generation auxiliary unit. The control unit controls to reduce the output of the induced current generation unit when it is determined that the heat generation auxiliary unit exceeds the Curie point based on the measurement result of the measurement unit.

1 is a side view of an image forming apparatus according to a first embodiment. The side view containing the control block of the IH coil unit and main body control circuit which concern on 1st Embodiment. The perspective view of the IH coil unit concerning a 1st embodiment. Explanatory drawing of the magnetic path to the fixing belt and the heat generating auxiliary plate by the magnetic flux of the main coil according to the first embodiment. The block diagram which shows the control system which mainly controls the IH coil unit which concerns on 1st Embodiment. FIG. 3 is a side view of a main part of the fixing device according to the first embodiment. 6 is a flowchart illustrating an example of the operation of the fixing device according to the first embodiment. FIG. 9 is a side view of a main part of a fixing device according to a second embodiment. FIG. 10 is a side view of a main part of a fixing device according to a third embodiment.

[First Embodiment]
Hereinafter, an image forming apparatus 10 according to the first embodiment will be described with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected about the same structure.

FIG. 1 is a side view of an image forming apparatus 10 according to the first embodiment. Hereinafter, the MFP 10 will be described as an example of the image forming apparatus 10.
As shown in FIG. 1, the MFP 10 includes a scanner 12, a control panel 13, and a main body 14. Each of the scanner 12, the control panel 13, and the main body unit 14 includes a control unit. The MFP 10 includes a system control unit 100 that is a control unit that controls each control unit. The system control unit 100 includes a CPU (Central Processing Unit) 100a, a ROM (Read Only Memory) 100b, and a RAM (Random Access Memory) 100c (see FIG. 5). The system control unit 100 controls a main body control circuit 101 (see FIG. 2) that is a control unit of the main body unit 14.
The main body control circuit 101 includes a CPU, a ROM, and a RAM (not shown). The main body unit 14 includes a paper feed cassette unit 16, a printer unit 18, a fixing device 34, and the like. The main body control circuit 101 controls the paper feed cassette unit 16, the printer unit 18, the fixing device 34, and the like.

  The scanner 12 reads a document image. The control panel 13 includes an input key 13a and a display unit 13b. For example, the input key 13a receives an input from the user. For example, the display unit 13b is a touch panel type. The display unit 13b accepts input from the user and performs display to the user.

The paper feed cassette unit 16 includes a paper feed cassette 16a and a pickup roller 16b. The paper feed cassette 16a stores sheets P that are recording media. The pickup roller 16b takes out the sheet P from the paper feed cassette 16a.
The paper feed cassette 16a feeds unused sheets P. The paper feed tray 17 feeds unused sheets P by a pickup roller 17a.

  The printer unit 18 forms an image. For example, the printer unit 18 forms an image of a document image read by the scanner 12. The printer unit 18 includes an intermediate transfer belt 21. The printer unit 18 supports the intermediate transfer belt 21 with a backup roller 40, a driven roller 41, and a tension roller 42. The backup roller 40 includes a drive unit (not shown). The printer unit 18 rotates the intermediate transfer belt 21 in the arrow m direction.

  The printer unit 18 includes four sets of image forming stations 22Y, 22M, 22C, and 22K. Each of the image forming stations 22Y, 22M, 22C, and 22K is for image formation of Y (yellow), M (magenta), C (cyan), and K (black), respectively. The image forming stations 22 </ b> Y, 22 </ b> M, 22 </ b> C and 22 </ b> K are arranged in parallel along the rotational direction of the intermediate transfer belt 21 below the intermediate transfer belt 21.

  The printer unit 18 includes cartridges 23Y, 23M, 23C, and 23K above the image forming stations 22Y, 22M, 22C, and 22K. Each of the cartridges 23Y, 23M, 23C, and 23K stores toner for supplying Y (yellow), M (magenta), C (cyan), and K (black).

  Hereinafter, among the image forming stations 22Y, 22M, 22C and 22K, the Y (yellow) image forming station 22Y will be described as an example. Since the image forming stations 22M, 22C, and 22K have the same configuration as the image forming station 22Y, detailed description thereof is omitted.

  The image forming station 22Y includes a charging charger 26, an exposure scanning head 27, a developing device 28, and a photoreceptor cleaner 29. The charging charger 26, the exposure scanning head 27, the developing device 28, and the photoreceptor cleaner 29 are arranged around the photoreceptor drum 24 that rotates in the direction of arrow n.

The image forming station 22Y includes a primary transfer roller 30. The primary transfer roller 30 faces the photosensitive drum 24 with the intermediate transfer belt 21 interposed therebetween.
In the image forming station 22 </ b> Y, the photosensitive drum 24 is charged by the charging charger 26 and then exposed by the exposure scanning head 27. The image forming station 22Y forms an electrostatic latent image on the photosensitive drum 24. The developing device 28 develops the electrostatic latent image on the photosensitive drum 24 using a two-component developer formed by toner and carrier.

  The primary transfer roller 30 primarily transfers the toner image formed on the photosensitive drum 24 to the intermediate transfer belt 21. The image forming stations 22Y, 22M, 22C, and 22K form a color toner image on the intermediate transfer belt 21 by the primary transfer roller 30. The color toner image is formed by sequentially superposing toner images of Y (yellow), M (magenta), C (cyan), and K (black). The photoreceptor cleaner 29 removes the toner remaining on the photoreceptor drum 24 after the primary transfer.

  The printer unit 18 includes a secondary transfer roller 32. The secondary transfer roller 32 faces the backup roller 40 through the intermediate transfer belt 21. The secondary transfer roller 32 secondary-transfers the color toner images on the intermediate transfer belt 21 to the sheet P at once. The sheet P is fed from the sheet feeding cassette unit 16 or the manual sheet feeding tray 17 along the conveyance path 33.

  The printer unit 18 includes a belt cleaner 43 that faces the driven roller 41 via the intermediate transfer belt 21. The belt cleaner 43 removes the toner remaining on the intermediate transfer belt 21 after the secondary transfer. The image forming unit includes an intermediate transfer belt 21, four sets of image forming stations (22Y, 22M, 22C, and 22K), and a secondary transfer roller 32.

The printer unit 18 includes a registration roller 33 a, a fixing device 34, and a paper discharge roller 36 along the conveyance path 33. The printer unit 18 includes a branching unit 37 and a reverse conveying unit 38 downstream of the fixing device 34. The branch unit 37 sends the fixed sheet P to the paper discharge unit 20 or the reverse conveyance unit 38. In the case of double-sided printing, the reverse conveying unit 38 conveys the sheet P sent from the branching unit 37 in the direction of the registration roller 33a. The MFP 10 forms a fixed toner image on the sheet P by the printer unit 18 and discharges it to the paper discharge unit 20.
The MFP 10 is not limited to the tandem developing method, and the number of developing devices 28 is not limited. The MFP 10 may directly transfer the toner image from the photosensitive drum 24 to the sheet P.

Hereinafter, the fixing device 34 will be described in detail.
FIG. 2 is a side view including control blocks of the electromagnetic induction heating coil unit 52 (induction current generating unit) and the main body control circuit 101 (control unit) according to the first embodiment. Hereinafter, the electromagnetic induction heating coil unit is referred to as “IH coil unit”.
As shown in FIG. 2, the fixing device 34 includes a fixing belt 50, a press roller 51, an IH coil unit 52, a heat generation auxiliary plate 69 (heat generation auxiliary unit), a second coil unit 84 (measurement unit), and a main body control circuit 101. Is provided.

  The fixing belt 50 is a cylindrical endless belt. A belt internal mechanism 55 including a nip pad 53 and a heat generating auxiliary plate 69 is disposed on the inner peripheral side of the fixing belt 50.

The fixing belt 50 is formed by sequentially laminating a heat generating layer 50a (conductive layer) as a heat generating portion and a release layer 50c on the base layer 50b (see FIG. 4). The fixing belt 50 is not limited in its layer structure as long as it includes the heat generating layer 50a.
For example, the base layer 50b is formed of a polyimide resin (PI). For example, the heat generating layer 50a is formed of a nonmagnetic metal such as copper (Cu). For example, the release layer 50c is formed of a fluororesin such as tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer resin (PFA).

In the fixing belt 50, the heat generation layer 50a is thinned to reduce the heat capacity in order to rapidly warm up. The fixing belt 50 having a small heat capacity shortens the time required for warming up and saves energy consumption.
For example, in the fixing belt 50, the thickness of the copper layer of the heat generating layer 50a is set to 10 μm in order to reduce the heat capacity. For example, the heat generating layer 50a is covered with a protective layer such as nickel. A protective layer such as nickel suppresses oxidation of the copper layer. The protective layer such as nickel improves the mechanical strength of the fixing belt 50.

  The heat generating layer 50a may be formed by performing electroless nickel plating and copper plating on the base layer 50b formed of polyimide resin. By applying electroless nickel plating, the adhesion strength between the base layer 50b and the heat generating layer 50a is improved. By applying electroless nickel plating, the mechanical strength of the fixing belt 50 is improved.

  Further, the surface of the base layer 50b may be roughened by sandblasting or chemical etching. By roughening the surface of the base layer 50b, the adhesion strength between the base layer 50b and the nickel plating of the heat generating layer 50a is further improved mechanically.

  Further, a metal such as titanium (Ti) may be dispersed in the polyimide resin forming the base layer 50b. By dispersing the metal in the base layer 50b, the adhesion strength between the base layer 50b and the nickel plating of the heat generating layer 50a is further improved.

  For example, the heat generating layer 50a may be formed of nickel, iron (Fe), stainless steel, aluminum (Al), silver (Ag), or the like. Two or more types of alloys may be used for the heat generating layer 50a, or two or more types of metals may be stacked in layers.

FIG. 3 is a perspective view of the IH coil unit 52 according to the first embodiment.
As shown in FIG. 3, the IH coil unit 52 includes a main coil 56 (first coil), a first core 57, and a second core 58.
The main coil 56 generates a magnetic flux when a high frequency current is applied. The main coil 56 is disposed on the outer peripheral side of the fixing belt 50. The main coil 56 faces the fixing belt 50 in the thickness direction. The main coil 56 has the longitudinal direction aligned with the width direction of the fixing belt 50 (hereinafter referred to as “belt width direction”).

The first core 57 and the second core 58 cover the side of the main coil 56 opposite to the fixing belt 50 (hereinafter referred to as “back side”). The first core 57 and the second core 58 prevent the magnetic flux generated by the main coil 56 from leaking to the back side. The first core 57 and the second core 58 concentrate the magnetic flux from the main coil 56 on the fixing belt 50.
The first core 57 includes a plurality of single wing portions 57a. The plurality of single wing portions 57a are alternately arranged in a zigzag manner with the center line 56d along the longitudinal direction of the main coil 56 being axially symmetric.

The second core 58 is disposed on both sides in the longitudinal direction of the first core 57. The second core 58 includes a plurality of wing portions 58 a that straddle both wings of the main coil 56.
For example, the single wing part 57a and the two wing parts 58a are formed of a magnetic material such as a nickel-zinc alloy (Ni-Zn) and a manganese-nickel alloy (Mn-Ni).

  The first core 57 regulates the magnetic flux generated by the main coil 56 by a plurality of single wing portions 57a. The magnetic flux generated by the main coil 56 is alternately regulated for each wing of the main coil 56 with the center line 56d being axially symmetrical. The first core 57 concentrates the magnetic flux from the main coil 56 on the fixing belt 50 by a plurality of single wing portions 57a.

  The second core 58 regulates the magnetic flux generated by the main coil 56 by the plurality of both wing portions 58a. The magnetic flux generated by the main coil 56 is regulated by both wings of the main coil 56 on both sides of the first core 57. The second core 58 concentrates the magnetic flux from the main coil 56 on the fixing belt 50 by the plurality of both wing portions 58a. The magnetic flux concentration force of the second core 58 is greater than the magnetic flux concentration force of the first core 57.

  The main coil 56 includes a first wing 56a and a second wing 56b. The first wing 56a is disposed on one side of the center line 56d. The second wing 56b is disposed on the other side across the center line 56d. A window portion 56 c is formed between the first wing 56 a and the second wing 56 b and inside the main coil 56 in the longitudinal direction.

  For example, the main coil 56 uses a litz wire. A litz wire is formed by bundling a plurality of copper wires covered with a heat-resistant polyamideimide that is an insulating material. The main coil 56 is formed around a conductive coil.

  As shown in FIG. 2, the main coil 56 generates a magnetic flux by application of a high-frequency current from the inverter drive circuit 68. For example, the inverter drive circuit 68 includes switching elements such as an IGBT (Insulated Gate Bipolar Transistor) element 68a and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) element (not shown). IGBT element 68a is connected to a MOSFET element. By alternately turning on and off the IGBT element 68a and the MOSFET element, a high-frequency current is passed through the main coil 56. By flowing a high frequency current through the main coil 56, a high frequency magnetic field is generated around the main coil 56. An eddy current is generated in the heat generating layer 50a of the fixing belt 50 by the magnetic flux of the high-frequency magnetic field. Joule heat is generated by the eddy current and the electric resistance of the heat generating layer 50a. The fixing belt 50 is heated by the generation of the Joule heat.

  For example, the ON period of the IGBT element 68a is assumed to be constant. By changing the ON period of the MOSFET element, the high-frequency current flowing through the main coil 56 changes. As the high-frequency current flowing through the main coil 56 changes, the output of electromagnetic induction heating changes.

  The heat generating auxiliary plate 69 is disposed on the inner peripheral side of the fixing belt 50. When viewed from the belt width direction, the heat generating auxiliary plate 69 is formed in an arc shape along the inner peripheral surface of the fixing belt 50. The heat generating auxiliary plate 69 faces the main coil 56 with the fixing belt 50 interposed therebetween. The heat generating auxiliary plate 69 has a magnetic material (ferromagnetic material) having a Curie point lower than that of the heat generating layer 50a. Magnetic flux is generated between the heat generating auxiliary plate 69 and the fixing belt 50 by the magnetic flux generated by the main coil 56. Due to the generation of the magnetic flux, Joule heat is generated in the heat generating layer 50 a of the fixing belt 50. The heating of the fixing belt 50 by the main coil 56 is assisted by the generation of the Joule heat.

  The auxiliary heat generating plate 69 is supported by a base (not shown) at both ends of the arc shape. The radially outer surface of the heat generating auxiliary plate 69 is separated from the inner peripheral surface of the fixing belt 50. For example, the distance between the radially outer surface of the heat generating auxiliary plate 69 and the inner peripheral surface of the fixing belt 50 is set to about 1 mm to 2 mm. Note that the radially outer surface of the heat generating auxiliary plate 69 may contact the inner peripheral surface of the fixing belt 50.

  For example, the length of the heat generating auxiliary plate 69 in the belt width direction is larger than the length of the sheet passing region in the belt width direction (hereinafter referred to as “sheet width”). The sheet width is a width of a sheet having the largest short side width among the sheets to be used. For example, the sheet width is assumed to be slightly larger than the short side width of A3 paper.

FIG. 4 is an explanatory diagram of a magnetic path to the fixing belt 50 and the heat generating auxiliary plate 69 by the magnetic flux of the main coil 56 according to the first embodiment.
As shown in FIG. 4, the magnetic flux generated by the main coil 56 forms a first magnetic path 81 that is guided to the heat generating layer 50 a of the fixing belt 50. The first magnetic path 81 is formed so as to surround the first wing 56 a and the second wing 56 b of the main coil 56. The first magnetic path 81 passes through the first core 57 and the second core 58 and the heat generating layer 50a. Further, the magnetic flux generated by the main coil 56 forms a second magnetic path 82 that is guided to the auxiliary heat generating plate 69. The second magnetic path 82 is formed at a position adjacent to the first magnetic path 81 in the radial direction of the fixing belt 50 (hereinafter referred to as “belt radial direction”). The second magnetic path 82 passes through the heat generation auxiliary plate 69 and the heat generation layer 50a.

  The heat generating auxiliary plate 69 is formed of a thin metal member made of a magnetic shunt alloy such as iron or nickel alloy having a Curie point of 220 ° C. to 230 ° C. The heat generating auxiliary plate 69 loses magnetism when it exceeds the Curie point. Specifically, the auxiliary heat generating plate 69 changes from ferromagnetic to paramagnetic when exceeding the Curie point. When the heat generating auxiliary plate 69 exceeds the Curie point, the second magnetic path 82 is not formed, and the heating of the fixing belt 50 is not assisted. By forming the heat generating auxiliary plate 69 of a magnetic shunt alloy, it is possible to suppress an excessive temperature increase of the fixing belt 50 at a high temperature while assisting the temperature increase of the fixing belt 50 at a low temperature with the Curie point as a boundary.

  Here, the heat generation of the fixing belt 50 is adjusted by power control by the IH control circuit 67. In order to keep the belt temperature constant, the IH coil unit 52 is controlled to a constant output. When the heat generating auxiliary plate 69 is formed of a magnetic shunt alloy, the heat generating auxiliary plate 69 loses magnetism when the Curie point is exceeded, and the second magnetic path 82 is not formed. If the second magnetic path 82 is not formed, the load (electric resistance) of the IH coil unit 52 decreases.

  The IH control circuit 67 increases the current flowing through the inverter drive circuit 68 by the decrease in the load of the IH coil unit 52, and tries to keep the output of the IH coil unit 52 constant. If the current flowing through the inverter drive circuit 68 increases, the current flowing through the IGBT element 68a also increases, so that the temperature of the IGBT element 68a may be excessively increased and the IGBT element 68a may be damaged. Therefore, in the present embodiment, as will be described later, the change in magnetism of the auxiliary heating plate 69 is estimated by measuring the electric resistance of the second coil 84a. Then, the IH control circuit 67 controls the IH coil unit 52 so as to reduce the output of electromagnetic induction heating when the measured electric resistance becomes less than the threshold value.

  The heat generating auxiliary plate 69 may be formed of a thin metal member having magnetic properties such as iron, nickel, and stainless steel. Further, the auxiliary heat generating plate 69 may be formed of a resin containing magnetic powder as long as it has magnetic characteristics. The auxiliary heat generating plate 69 may be formed of the following magnetic material (ferrite). The magnetic material (ferrite) promotes heat generation of the fixing belt 50 through a magnetic flux caused by an induced current. Magnetic materials (ferrites) do not self-heat even when exposed to magnetic flux due to induced current. The heat generating auxiliary plate 69 is not limited to a thin plate member.

  As shown in FIG. 2, the shield 76 is disposed on the inner peripheral side of the heat generating auxiliary plate 69. The shield 76 is formed in an arc shape similar to the heat generation auxiliary plate 69. The shield 76 is supported by a base (not shown) at both ends of the arc shape. The shield 76 may support the heat generating auxiliary plate 69. For example, the shield 76 is formed of a nonmagnetic material such as aluminum and copper. The shield 76 shields the magnetic flux from the IH coil unit 52. The shield 76 suppresses the magnetic flux from affecting the measurement voltage of the thermistor.

  The nip pad 53 is a pressing portion that presses the inner peripheral surface of the fixing belt 50 toward the press roller 51. A nip 54 is formed between the fixing belt 50 and the press roller 51. The nip pad 53 has a nip forming surface 53 a that forms a nip 54 between the fixing belt 50 and the press roller 51. The nip forming surface 53a is curved so as to protrude toward the inner peripheral side of the fixing belt 50 when viewed from the belt width direction. The nip forming surface 53a is curved along the outer peripheral surface of the press roller 51 when viewed from the belt width direction.

  For example, the press roller 51 includes a heat-resistant silicon sponge and a silicon rubber layer around the cored bar. For example, a release layer is disposed on the surface of the press roller 51. The release layer is formed of a fluorine resin such as PFA resin. The press roller 51 presses the fixing belt 50 by a pressing mechanism. The press roller 51 is a pressure unit that presses the fixing belt 50 together with the nip pad 53.

  One motor 51 b is provided as a drive source for the fixing belt 50 and the press roller 51. The motor 51b is driven by a motor drive circuit 51c controlled by the main body control circuit 101. The motor 51b is connected to the press roller 51 via a first gear train (not shown). The motor 51b is connected to a belt drive member (both not shown) via a second gear train and a one-way clutch. The press roller 51 is rotated in the direction of arrow q by the motor 51b. When the fixing belt 50 and the press roller 51 are in contact with each other, the fixing belt 50 is driven by the press roller 51 and rotates in the arrow u direction. When the fixing belt 50 and the press roller 51 are separated, the fixing belt 50 is rotated in the arrow u direction by the motor 51b. Note that the fixing belt 50 may include a drive source independently of the press roller 51.

  The center thermistor 61 and the edge thermistor 62 measure the belt temperature. The measurement result of the belt temperature is input to the main body control circuit 101. The center thermistor 61 is disposed on the inner side in the belt width direction. The edge thermistor 62 is disposed in the heating area and the non-sheet passing area of the IH coil unit 52 in the belt width direction. The main body control circuit 101 controls the IH coil unit 52 so as to stop the output of electromagnetic induction heating when the belt temperature measured by the edge thermistor 62 becomes equal to or higher than a threshold value. Damage to the fixing belt 50 is prevented by stopping the output of electromagnetic induction heating when the non-sheet passing area of the fixing belt 50 is excessively heated.

  The main body control circuit 101 controls the IH control circuit 67 according to the belt temperature measurement results of the center thermistor 61 and the edge thermistor 62. The IH control circuit 67 controls the magnitude of the high-frequency current output from the inverter drive circuit 68 under the control of the main body control circuit 101. The fixing belt 50 maintains various control temperature ranges according to the output of the inverter drive circuit 68. The IH control circuit 67 includes a CPU, a ROM, and a RAM (not shown).

  The thermostat 63 functions as a safety device for the fixing device 34. The thermostat 63 operates when the fixing belt 50 abnormally generates heat and the temperature rises to the cutoff threshold. By the operation of the thermostat 63, the current to the IH coil unit 52 is interrupted. By shutting off the current to the IH coil unit 52, the MFP 10 stops driving and suppresses abnormal heat generation of the fixing device 34.

Hereinafter, the control system 110 of the IH coil unit 52 that generates heat from the fixing belt 50 will be described in detail.
FIG. 5 is a block diagram showing a control system 110 that mainly controls the IH coil unit 52 according to the first embodiment.
As shown in FIG. 5, the control system 110 includes a system control unit 100, a main body control circuit 101, an IH circuit 120, and a motor drive circuit 51c.

  The control system 110 supplies power to the IH coil unit 52 by the IH circuit 120. The IH circuit 120 includes a rectifier circuit 121, an IH control circuit 67, an inverter drive circuit 68, and a current measurement circuit 122.

  The IH circuit 120 receives a current from the AC power supply 111 via the relay 112. The IH circuit 120 rectifies the input current by the rectifier circuit 121 and supplies the rectified current to the inverter drive circuit 68. The relay 112 cuts off the current from the AC power supply 111 when the thermostat 63 is disconnected. The inverter drive circuit 68 includes a drive IC 68b of the IGBT element 68a. The IH control circuit 67 controls the drive IC 68b according to the measurement result of the belt temperature by the center thermistor 61 and the edge thermistor 62. The IH control circuit 67 controls the drive IC 68b to control the output of the IGBT element 68a. The current measurement circuit 122 sends the measurement result of the output of the IGBT element 68 a to the IH control circuit 67. Based on the measurement result of the output of the IGBT element 68a by the current measurement circuit 122, the IH control circuit 67 controls the drive IC 68b so that the output of the IH coil unit 52 becomes constant.

  The main body control circuit 101 acquires a measurement value R (see FIG. 7) described later from the electrical resistance measurement circuit 84b. The main body control circuit 101 controls the IH coil unit 52 based on the measured value R. The main body control circuit 101 determines whether or not the measured value R is less than the threshold value Rt. The main body control circuit 101 controls either continuation of driving of the fixing device 34 or reduction of the output of the IH coil unit 52 based on the determination result. Note that the reduction of the output of the IH coil unit 52 includes stopping of the IH coil unit 52.

FIG. 6 is a side view of a main part of the fixing device 34 according to the first embodiment.
As shown in FIG. 6, the second coil unit 84 includes a second coil 84a and an electric resistance measuring circuit 84b (electric resistance measuring unit). The second coil unit 84 measures whether or not the heat generating auxiliary plate 69 is in a state exceeding the Curie point. The second coil 84 a is configured separately from the main coil 56. The second coil 84a generates a magnetic field that passes through the auxiliary heat generating plate 69 when energized. For example, the second coil 84a uses a litz wire winding. The electrical resistance measurement circuit 84b measures the electrical resistance of the second coil 84a. The measurement result of the electrical resistance of the second coil 84 a is input to the main body control circuit 101.

Hereinafter, in the heat generating auxiliary plate 69, a region facing the IH coil unit 52 across the fixing belt 50 in the circumferential direction of the fixing belt 50 (hereinafter referred to as “belt circumferential direction”) is referred to as a facing region 69 a. An end portion 69c of the heat generation auxiliary plate 69 is an end portion in the belt circumferential direction of the heat generation auxiliary plate 69 and is a region adjacent to the facing region 69a. The end portion 69c of the heat generating auxiliary plate 69 does not face the IH coil unit 52 across the fixing belt 50 in the belt radial direction.
The end portion 52c of the IH coil unit 52 is an end portion in the belt circumferential direction of the first core 57 and the second core 58, and includes a region protruding inward in the belt radial direction.

  The second coil 84a is disposed in a region S1 (see FIG. 2) that faces the heat generating auxiliary plate 69 and does not face the main coil 56. Specifically, the region S1 is located between the end portion 52c of the IH coil unit 52 and the fixing belt 50 in the belt radial direction. The region S1 is a range from the outside of the main coil 56 to the end portion 69c of the heat generating auxiliary plate 69 in the belt circumferential direction. The region S1 faces the end portion 52c of the IH coil unit 52 in the belt circumferential direction, and faces the end portion 69c of the heat generating auxiliary plate 69 with the fixing belt 50 interposed therebetween. One end (inner end) in the belt circumferential direction in the region S1 faces the boundary between the end 52c of the IH coil unit 52 and the main coil 56 in the belt radial direction. The other end (outer end) in the belt width direction in the region S1 faces the front end (arc-shaped both ends) of the end portion 69c of the auxiliary heat generating plate 69 across the fixing belt 50 in the belt radial direction.

In the present embodiment, the second coil 84 a is disposed on the outer peripheral side of the fixing belt 50. The second coil 84 a faces the end portion 69 c of the heat generating auxiliary plate 69 with the fixing belt 50 interposed therebetween.
Note that the second coil 84 a may face the facing region 69 a of the auxiliary heat generating plate 69 with the fixing belt 50 interposed between the second coil 84 a and the main coil 56.

  The second coil 84a is fixed at a predetermined distance from the fixing belt 50. The second coil 84a faces at least the paper passing area in the belt width direction. For example, the second coil 84 a faces the central portion of the fixing belt 50.

The size of the second coil 84 a is smaller than that of the main coil 56. This is because it suffices if the second coil 84a generates a magnetic field that passes through the heat generating auxiliary plate 69 by energization, and the electric resistance measurement circuit 84b can measure the electric resistance of the second coil 84a.
In addition, compared with the case where the magnitude | size of the 2nd coil 84a shall be equal to or more than the main coil 56, the 2nd coil 84a can be made easy to arrange | position in the said area | region S1.

  The magnetic flux generated by the second coil 84 a forms a third magnetic path 85 that is guided to the heat generating layer 50 a of the fixing belt 50. The third magnetic path 85 passes through the heat generating layer 50a. Further, the magnetic flux generated by the second coil 84a forms a fourth magnetic path 86 that is guided to the auxiliary heat generating plate 69 before the auxiliary heat generating plate 69 loses magnetism beyond the Curie point. The fourth magnetic path 86 is formed at a position adjacent to the third magnetic path 85 in the belt radial direction. The fourth magnetic path 86 passes through the heat generation auxiliary plate 69 and the heat generation layer 50a. The electrical resistance of the second coil 84a changes as the magnetism of the auxiliary heating plate 69 changes. That is, the electric resistance of the second coil 84a changes depending on whether or not the fourth magnetic path 86 is formed.

  By passing a weak high-frequency current (hereinafter referred to as “high-frequency weak current”) through the second coil 84a, the electrical resistance of the second coil 84a can be measured. For example, the electrical resistance measurement circuit 84b is connected to the upstream side and the downstream side of the second coil 84a, and the electrical resistance is measured from the current values on the upstream side and the downstream side of the second coil 84a. The high frequency weak current is weaker than the high frequency current output from the inverter drive circuit 68.

Next, an example of the operation of the fixing device 34 according to the first embodiment will be described with reference to FIG.
FIG. 7 is a flowchart illustrating an example of the operation of the fixing device 34 according to the first embodiment.

  In Act 100, the electrical resistance measurement circuit 84b passes a high-frequency weak current through the second coil 84a. For example, the high frequency weak current is set to a frequency of 60 kHz and a current of 10 mA.

In Act 101, the electrical resistance measurement circuit 84b measures the electrical resistance of the second coil 84a. In the present embodiment, the electrical resistance of the second coil 84a measured by the electrical resistance measurement circuit 84b is referred to as “measured value R”. The main body control circuit 101 acquires the measurement value R from the electrical resistance measurement circuit 84b.
The main body control circuit 101 may acquire the measurement value R via another circuit such as a logic circuit.

  In Act 102, the main body control circuit 101 determines whether or not the measurement value R acquired in Act 101 is less than a threshold value Rt (for example, 1 [Ω]).

By determining whether or not the measured value R is less than the threshold value Rt for the following reason, it is possible to determine the change in magnetism of the heat generating auxiliary plate 69.
When the measured value R is greater than or equal to the threshold value Rt, the heat generating auxiliary plate 69 has ferromagnetism without exceeding the Curie point. When the heat generating auxiliary plate 69 has ferromagnetism, the magnetic flux generated by the second coil 84 a forms a third magnetic path 85 and a fourth magnetic path 86.
On the other hand, when the measured value R becomes less than the threshold value Rt, the heat generating auxiliary plate 69 changes to paramagnetic beyond the Curie point. When the heat generating auxiliary plate 69 changes to paramagnetism, the fourth magnetic path 86 is lost.
Therefore, by determining whether or not the measured value R is less than the threshold value Rt, it can be estimated that the magnetism of the heat generating auxiliary plate 69 has changed.

  When determining that the measured value R is less than the threshold value Rt (Act 102; Yes), the main body control circuit 101 advances the process to Act 104. If the main body control circuit 101 determines that the measured value R is greater than or equal to the threshold value Rt (Act 102; No), the main control circuit 101 advances the process to Act 103.

  In Act 103, the fixing device 34 continues to be driven. For example, the fixing device 34 continues the high output driving when the high output driving such as continuous paper passing and warming up is performed.

  In Act 104, the main body control circuit 101 controls the IH coil unit 52 based on the measurement value R. Control by the main body control circuit 101 based on the measured value R is performed for the following reason.

In Act 105, the main body control circuit 101 determines whether to stop the IH coil unit 52 based on the measured value R. For example, the main body control circuit 101 determines to stop the IH coil unit 52 when the measured value R is less than 0.5 [Ω]. When it is determined that the IH coil unit 52 is to be stopped (Act 105; Yes), the main body control circuit 101 ends the process. The main body control circuit 101 suppresses the temperature of the IGBT element 68a from excessively rising by stopping the IH coil unit 52. The main body control circuit 101 prevents the IGBT element 68a from being damaged by suppressing the temperature of the IGBT element 68a from rising excessively.
When determining that the IH coil unit 52 is not stopped (Act 105; No), the main body control circuit 101 advances the process to Act 106.

In Act 106, the main body control circuit 101 reduces the output of the IH coil unit 52. For example, the main body control circuit 101 reduces the power supplied to the IH coil unit 52. The main body control circuit 101 suppresses an excessive increase in the temperature of the IGBT element 68a by reducing the output of the IH coil unit 52. The main body control circuit 101 prevents the IGBT element 68a from being damaged by suppressing the temperature of the IGBT element 68a from rising excessively.
In Act 103, the fixing device 34 continues driving with the output of the IH coil unit 52 reduced.

Hereinafter, the operation of the fixing device 34 during warm-up will be described.
As shown in FIG. 2, during warming up, the fixing device 34 rotates the fixing belt 50 in the arrow u direction. The IH coil unit 52 generates a magnetic flux on the fixing belt 50 side when a high frequency current is applied by the inverter drive circuit 68.

For example, at the time of warming up, the fixing belt 50 is rotated in the arrow u direction with the fixing belt 50 separated from the press roller 51. By rotating the fixing belt 50 away from the press roller 51 at the time of warming up, the following effects can be obtained. Compared to the case where the fixing belt 50 rotates while being in contact with the press roller 51, it is possible to avoid the heat of the fixing belt 50 being taken away by the press roller 51. By avoiding the heat of the fixing belt 50 being taken away by the press roller 51, the warm-up time can be shortened.
During the warm-up, the fixing belt 50 may be driven to rotate in the direction of the arrow u by rotating the press roller 51 in the direction of the arrow q while the press roller 51 is in contact with the fixing belt 50.

  As shown in FIG. 4, the IH coil unit 52 heats the fixing belt 50 by the first magnetic path 81. The heat generating auxiliary plate 69 assists the heating of the fixing belt 50 by the second magnetic path 82. By assisting heating of the fixing belt 50, rapid warm-up of the fixing belt 50 is promoted.

  As shown in FIG. 2, the IH control circuit 67 controls the inverter drive circuit 68 from the measurement result of the belt temperature by the center thermistor 61 or the edge thermistor 62. The inverter drive circuit 68 supplies a high frequency current to the main coil 56.

Hereinafter, the operation of the fixing device 34 during the fixing operation will be described.
After the fixing belt 50 reaches the fixing temperature and finishes warming up, the press roller 51 is brought into contact with the fixing belt 50. With the press roller 51 in contact with the fixing belt 50, the fixing roller 50 is driven to rotate in the direction of arrow u by rotating the press roller 51 in the direction of arrow q. When there is a print request, the MFP 10 (see FIG. 1) starts a print operation. The MFP 10 forms a toner image on the sheet P by the printer unit 18 and conveys the sheet P to the fixing device 34.

  The MFP 10 passes the sheet P on which the toner image is formed through the nip 54 between the fixing belt 50 that has reached the fixing temperature and the press roller 51. The fixing device 34 fixes the toner image on the sheet P. During fixing, the IH control circuit 67 controls the IH coil unit 52 to keep the fixing belt 50 at the fixing temperature.

  The fixing belt 50 is deprived of heat by the sheet P by the fixing operation. For example, when the sheet is continuously fed at a high speed, since the heat amount taken by the sheet P is large, the fixing belt 50 having a low heat capacity may not be able to maintain the fixing temperature. The heating assist of the fixing belt 50 by the second magnetic path 82 compensates for the shortage of the belt heat generation amount. The heating assist of the fixing belt 50 by the second magnetic path 82 maintains the belt temperature at the fixing temperature even during continuous high-speed paper feeding.

  By the way, in order to prevent the IGBT element 68a from being damaged, it is conceivable to provide a thermistor for measuring the temperature of the IGBT element 68a. The thermistor is attached not to the IGBT element 68a itself but to the case of the inverter drive circuit 68. When the thermistor measures the temperature rise of the IGBT element 68a, the main body control circuit 101 drives the fan to cool the IGBT element 68a. According to the thermistor, the moderate temperature rise of the IGBT element 68a can be measured. However, it is difficult for the thermistor to measure a steep temperature rise, and the temperature followability of the IGBT element 68a is limited. Further, since the thermistor is attached to the case, it is difficult for the thermistor to measure the exact temperature of the IGBT element 68a. The measured temperature of the IGBT element 68a by the thermistor may deviate from the actual internal temperature of the IGBT element 68a. In addition, with the cooling of the IGBT element 68a by the fan, it is difficult to cool the inside of the IGBT element 68a, and there is a limit to sufficiently cooling the IGBT element 68a. Therefore, there is a possibility that damage to the IGBT element 68a cannot be prevented by temperature measurement with a thermistor and cooling with a fan.

On the other hand, according to the first embodiment, the electrical resistance measurement circuit 84b measures the electrical resistance of the second coil 84a. By measuring the electric resistance of the second coil 84a, it is possible to indirectly measure a gradual temperature increase and a steep temperature increase of the IGBT element 68a. By measuring the electrical resistance of the second coil 84a, the temperature of the IGBT element 68a can be indirectly measured in real time as compared with the case where the thermistor is provided. Further, by measuring the electric resistance of the second coil 84a, the above-described deviation from the internal temperature of the IGBT element 68a does not matter.
In addition, the main body control circuit 101 acquires the electrical resistance (measured value R) of the second coil 84a from the electrical resistance measurement circuit 84b. The main body control circuit 101 controls the IH coil unit 52 so as to reduce the output of the electromagnetic induction heating when the measured value R becomes less than the threshold value. By reducing the output of the electromagnetic induction heating when the measured value R becomes less than the threshold value, it is possible to suppress the temperature of the IGBT element 68a from rising excessively. Specifically, the main body control circuit 101 determines whether or not the measured value R is less than the threshold value Rt. When it is determined that the measured value R is less than the threshold value Rt, the main body control circuit 101 can reduce the output of the IH coil unit 52. For example, by stopping the IH coil unit 52 or reducing the output of the IH coil unit 52, it is possible to prevent the temperature of the IGBT element 68a from rising excessively. Therefore, damage to the IGBT element 68a can be prevented.

  Further, since the second coil 84a is configured separately from the main coil 56, the electric resistance measurement circuit 84b can always measure the electric resistance of the second coil 84a. Therefore, the main body control circuit 101 can always acquire the measurement value R.

  Further, the second coil 84a is disposed in the region S1 facing the heat generating auxiliary plate 69 and not facing the main coil 56, thereby providing the following effects. Compared with the case where the second coil 84a is disposed in a region facing the main coil 56, the second coil 84a can be suppressed from being influenced by the large magnetic force of the main coil 56, and therefore the second coil 84a Electrical resistance can be measured with high accuracy.

  Further, the second coil 84a faces the end portion 69c (a part adjacent to the facing region 69a) of the heat generating auxiliary plate 69 with the fixing belt 50 interposed therebetween, and the following effects are obtained. The second coil unit 84 can measure the electrical resistance of the second coil 84a at a place having a temperature change equivalent to that of the facing area 69a (a place having a correlation with the temperature change of the facing area 69a).

  Further, since the second coil 84a faces at least the paper passing area in the belt width direction, the second coil unit 84 can be separated from the non-paper passing area and measure the electric resistance of the second coil 84a. it can. Therefore, the main body control circuit 101 can acquire the measurement value R separately from the non-sheet passing area.

[Second Embodiment]
Next, a second embodiment will be described based on FIG. In addition, the same code | symbol is attached | subjected to the same aspect as 1st Embodiment, and description is abbreviate | omitted.
FIG. 8 is a side view of a main part of the fixing device 234 according to the second embodiment. FIG. 8 is a side view corresponding to FIG.

  As shown in FIG. 8, the fixing device 234 according to the second embodiment does not include the second coil 84a according to the first embodiment. The fixing device 234 according to the second embodiment is different from the first embodiment described above in that it includes a measurement unit 284 using the main coil 56. In FIG. 8, reference numeral 284b denotes an electric resistance measurement circuit.

The IH coil unit 52 includes a main coil 56 (coil) that electromagnetically heats the heat generating layer 50a. The IH coil unit 52 also functions as the measurement unit 284. The measuring unit 284 generates a magnetic field that passes through the auxiliary heat generating plate 69 by energizing the main coil 56. The measurement unit 284 measures the electrical resistance of the main coil 56.
The magnetic flux generated by the main coil 56 forms a first magnetic path 81 and a second magnetic path 82. The electrical resistance of the main coil 56 changes with the change in magnetism of the heat generating auxiliary plate 69.

By passing a high frequency weak current through the main coil 56, the electrical resistance of the main coil 56 can be measured.
The electrical resistance measurement circuit 284 b measures the electrical resistance of the main coil 56. In the present embodiment, the electrical resistance of the main coil 56 measured by the electrical resistance measurement circuit 284b is referred to as “measured value R”. The main body control circuit 101 acquires the measurement value R from the electrical resistance measurement circuit 284b.

  The main body control circuit 101 determines whether or not the acquired measurement value R is less than a threshold value Rt (for example, 1 [Ω]).

By determining whether or not the measured value R is less than the threshold value Rt for the following reason, it is possible to determine the change in magnetism of the heat generating auxiliary plate 69.
When the measured value R is greater than or equal to the threshold value Rt, the heat generating auxiliary plate 69 has ferromagnetism without exceeding the Curie point. When the heat generating auxiliary plate 69 has ferromagnetism, the magnetic flux generated by the main coil 56 forms a first magnetic path 81 and a second magnetic path 82.
On the other hand, when the measured value R becomes less than the threshold value Rt, the heat generating auxiliary plate 69 changes to paramagnetic beyond the Curie point. When the heat generating auxiliary plate 69 changes to paramagnetism, the second magnetic path 82 is lost.
Therefore, by determining whether or not the measured value R is less than the threshold value Rt, it can be estimated that the magnetism of the heat generating auxiliary plate 69 has changed.

  The main body control circuit 101 controls the IH coil unit 52 so as to reduce the output of electromagnetic induction heating when the acquired measured value R becomes less than the threshold value Rt.

According to the second embodiment, the same effect as in the first embodiment can be obtained.
Further, as compared with the case where the second coil 84a faces the end portion 69c of the auxiliary heat generating plate 69 with the fixing belt 50 interposed therebetween, the electrical resistance of the main coil 56 can be measured at the place facing the facing region 69a. . Therefore, it is possible to determine the change in magnetism of the facing region 69a.
Further, the electrical resistance of the main coil 56 can be measured at a timing when the IH coil unit 52 does not generate heat. For example, the electrical resistance of the main coil 56 can be measured between jobs other than during continuous paper feeding and warming up (for example, every 10 sheets passed). Therefore, it is possible to determine the change in magnetism of the facing area 69a between jobs.
In addition, since the number of parts can be reduced as compared with the case where the second coil 84a is configured separately from the main coil 56, the configuration of the fixing device 234 can be simplified.
The electrical resistance of the main coil 56 may be measured at the timing when the IH coil unit 52 generates heat. For example, the electric resistance of the main coil 56 may be measured during continuous paper feeding and warming up. By measuring the electric resistance of the main coil 56 during continuous paper feeding and warming up, the electric resistance of the main coil 56 can be measured in real time at a location facing the facing area 69a. Accordingly, it is possible to determine the change in magnetism of the facing region 69a in real time during continuous paper feeding and warming up.

[Third Embodiment]
Next, a third embodiment will be described based on FIG. In addition, the same code | symbol is attached | subjected to the same aspect as 1st Embodiment, and description is abbreviate | omitted.
FIG. 9 is a side view of a main part of the fixing device 334 according to the third embodiment. FIG. 9 is a side view corresponding to FIG.

  As shown in FIG. 9, the fixing device 334 according to the third embodiment does not include the second coil 84a according to the first embodiment. The fixing device 334 according to the third embodiment is different from the above-described first embodiment in that it includes a second coil 384 a disposed on the inner peripheral side of the fixing belt 50. The second coil 384 a is disposed on the radially inner side of the heat generating auxiliary plate 69. In FIG. 9, reference numeral 384 denotes a second coil unit, and reference numeral 384b denotes an electric resistance measurement circuit.

The magnetic flux generated by the second coil 384a forms a fifth magnetic path 87 that is guided to the heat generating auxiliary plate 69 before the heat generating auxiliary plate 69 loses magnetism beyond the Curie point. The fifth magnetic path 87 passes through the heat generation auxiliary plate 69 without protruding outward in the belt radial direction from the heat generation auxiliary plate 69.
The magnetic flux generated by the second coil 384a forms a sixth magnetic path 88 that is guided to the heat generating layer 50a of the fixing belt 50 when the heat generating auxiliary plate 69 loses magnetism beyond the Curie point. . The sixth magnetic path 88 protrudes outward in the belt radial direction from the heat generation auxiliary plate 69 and passes through the heat generation layer 50a. The electrical resistance of the second coil 384a changes as the magnetism of the auxiliary heat generating plate 69 changes.

The electric resistance of the second coil 384a can be measured by flowing a high-frequency weak current through the second coil 384a.
The electrical resistance measurement circuit 384b measures the electrical resistance of the second coil 384a. In the present embodiment, the electrical resistance of the second coil 384a measured by the electrical resistance measurement circuit 384b is defined as “measured value R”. The main body control circuit 101 acquires the measurement value R from the electrical resistance measurement circuit 384b.

  The main body control circuit 101 determines whether or not the acquired measurement value R is less than a threshold value Rt (for example, 1 [Ω]).

By determining whether or not the measured value R is less than the threshold value Rt for the following reason, it is possible to determine the change in magnetism of the heat generating auxiliary plate 69.
When the measured value R is greater than or equal to the threshold value Rt, the heat generating auxiliary plate 69 has ferromagnetism without exceeding the Curie point. When the heat generating auxiliary plate 69 has ferromagnetism, the magnetic flux generated by the second coil 384 a forms a fifth magnetic path 87.
On the other hand, when the measured value R becomes less than the threshold value Rt, the heat generating auxiliary plate 69 changes to paramagnetic beyond the Curie point. When the heat generating auxiliary plate 69 changes to paramagnetism, the magnetic flux generated by the second coil 384a forms a sixth magnetic path 88. When the heat generating auxiliary plate 69 is changed to paramagnetic, the fifth magnetic path 87 is lost.
Therefore, by determining whether or not the measured value R is less than the threshold value Rt, it can be estimated that the magnetism of the heat generating auxiliary plate 69 has changed.

  The main body control circuit 101 controls the IH coil unit 52 so as to reduce the output of electromagnetic induction heating when the acquired measured value R becomes less than the threshold value Rt.

According to the third embodiment, the same effect as in the first embodiment can be obtained.
In addition, by arranging the second coil 384a on the radially inner side of the heat generating auxiliary plate 69 on the inner peripheral side of the fixing belt 50, the following effects can be obtained. Compared to the case where the second coil 84 a is disposed on the outer peripheral side of the fixing belt 50, the second coil 384 a can be integrated together with the heat generating auxiliary plate 69 on the inner peripheral side of the fixing belt 50.

  According to the fixing device of at least one embodiment described above, it is possible to prevent the temperature of the IGBT element 68a from rising excessively, and thus it is possible to prevent the IGBT element 68a from being damaged.

The heating layer 50a described above may be formed of a magnetic material such as nickel.
Moreover, the measurement part mentioned above is not restricted to providing an electrical resistance measurement part. For example, the measurement unit may include a temperature measurement unit that measures the temperature of the heat generation auxiliary plate 69. For example, the temperature measurement unit uses a temperature sensor. By measuring the temperature of the heat generating auxiliary plate 69, it is possible to directly determine whether or not the heat generating auxiliary plate 69 has exceeded the Curie point. That is, the measurement unit may measure the state of the heat generation auxiliary plate 69.
Further, the main body control circuit 101 described above is not limited to indirectly determining whether or not the heat generation auxiliary plate 69 exceeds the Curie point based on the measurement result of the electric resistance measurement circuit. For example, the main body control circuit 101 may directly determine whether or not the heat generation auxiliary plate 69 exceeds the Curie point based on the measurement result of the temperature sensor. That is, the main body control circuit 101 may control to reduce the output of the IH coil unit when determining that the heat generation auxiliary plate 69 exceeds the Curie point based on the measurement result of the measurement unit.

  You may make it implement | achieve the function of the fixing device in embodiment mentioned above with a computer. In that case, a program for realizing this function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Furthermore, the “computer-readable recording medium” dynamically holds a program for a short time like a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program that holds a program for a certain period of time. The program may be a program for realizing a part of the functions described above, and may be a program capable of realizing the functions described above in combination with a program already recorded in a computer system.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 10 ... Image forming apparatus, 34,234,334 ... Fixing device, 50 ... Fixing belt, 50a ... Heat-generating layer (conductive layer), 52 ... IH coil unit (inductive current generator), 56 ... Main coil (coil, first , 69... Heat generation auxiliary plate (heat generation auxiliary portion), 84, 384... Second coil unit (measurement portion), 84 a, 384 a... Second coil, 101... Main body control circuit (control portion) 284. Measuring unit, S1 ... area

Claims (2)

A fixing belt comprising a conductive layer;
An induced current generator having a first coil facing the fixing belt and electromagnetically heating the conductive layer;
A heat generating auxiliary part facing the induced current generating part across the fixing belt and having a Curie point of 220 ° C. to 230 ° C . ;
A measurement unit for measuring whether or not the heat generation auxiliary unit is in a state exceeding the Curie point ;
A control unit that controls to reduce the power supplied to the induced current generation unit when it is determined that the heat generation auxiliary unit exceeds the Curie point based on the measurement result of the measurement unit ;
The measuring unit is
A second coil that is disposed in a region that faces the heat generating auxiliary part and does not face the first coil, and generates a magnetic field that passes through the heat generating auxiliary part;
An electrical resistance measurement unit for measuring electrical resistance of the second coil,
The control unit determines that the heat generation assisting unit does not exceed the Curie point when the electrical resistance is equal to or greater than a threshold value, and determines that the heat generation assisting unit exceeds the Curie point when the electrical resistance is less than the threshold value. Fixing device. The first coil is disposed on the outer peripheral side of the fixing belt,
The heating auxiliary part and the second coil are arranged on the inner peripheral side of the fixing belt,
The fixing device according to claim 1 , wherein the second coil is disposed radially inward of the fixing belt with respect to the heat generation assisting unit.

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